The present invention relates to an FM stereo demodulating circuit and particularly to an FM stereo demodulating circuit in which a composite signal is multiplied by a subcarrier signal in demodulating a sub-signal.
A circuit is known for demodulating an FM stereo signal in which a composite signal is subjected to switching by a rectangular subcarrier signal of 38 kHz to separately extract a left and right channel signal from the composite signal. FIG. 1 is a block diagram illustrating such a demodulating system, in which an FM-IF (intermediate frequency) signal is converted into a composite signal by an FM detector 101 and the composite signal is applied to a switching circuit 103 through a low-pass filter (LPF) 102 for eliminating unnecessary components. A pilot signal of 19 kHz contained in the output of the LPF 102 is extracted by a phase-locked loop (PLL) circuit 104. A rectangular subcarrier signal of 38 kHz which is phase-synchronized with the pilot signal is used as a switching signal for the switching circuit 103. LPFs 105 and 106 are used to derive the two audio components from the output of the switching circuit, that is, separate left and right channel signals.
The subcarrier signal of 38 kHz, which is used as the switching signal, is a rectangular wave as shown in FIG. 2(A) and therefore can be expanded in a Fourier series as follows: ##EQU1## where .omega..sub.s designates the angular frequency of the subcarrier signal. Thus, the frequency spectrum of the signal F(t) contains odd order harmonics such as 114 kHz, 190 kHz, etc. higher than the fundamental wave of 38 kHz, as shown in the power spectrum of FIG. 2(B).
By switching the FM detection output by the switching signal F(t) having such a frequency spectrum as mentioned above, multiplication is performed on both the signals. If each of the LPFs 5 and 6 is arranged to have a passband between 0 and 15 kHz, the detector output appearing at the stereo output as a result of this multiplication is as shown in FIG. 2(C). Accordingly, signals other than the main signal of 0-15 kHz and the sub-signal of 39.+-.15 kHz, such as noise or nearby electromagnetic disturbances at 114.+-.15 kHz, 190.+-.15 kHz, etc. are demodulated and outputted.
In order to eliminate such a disadvantage, it is necessary to additionally provide an LPF having a large attenuation around the harmonic frequencies of 114 kHz, 190 kHz, etc., as shown in FIG. 2(D) at the output of the FM detector 101. Since the frequency 114 kHz is close to the frequency of the composite signal, there is a disadvantage that the flatness of the delay characteristic of the composite signal is degraded as shown in FIG. 2(E) or the flatness of the amplitude characteristic is degraded due to the inclusion of the additional LPF.